This invention relates to solid state microwave amplifiers and more particularly to divider/combiner circuits for obtaining higher power output than can be obtained from one solid state amplifier by appropriately combining the output power from more than one amplifier. More particularly, the circumferential divider/combiner circuit of the invention combines the output power of a moderate number of high frequency bipolar and/or field effect transistors to provide high power amplification in the 8-20 GHz frequency band. In this frequency range, power amplification techniques are almost totally dominated by thermionic-cathode microwave tubes with some limited applications for one-port negative resistance semiconductor devices. The need for higher-power solid state microwave amplifiers exists in order to provide amplifiers of smaller size, lighter weight, increased reliability and lower cost than are presently available.
In the prior art, the semiconductor devices which are available for amplification in the 10 GHz frequency range are limited in the output power that they can provide. Thus, although they have a broad bandwidth and have the advantage of not utilizing thermionic cathodes, their lack of ability to produce high power is a substantial limitation to their application. These active semiconductor devices have been incorporated into prior art circuits to increase their output power by paralleling a number of devices. However, it has been found that paralelling of individual semiconductor devices has disadvantages in reduction of efficiency and the effect of paralleling upon the impedance at the input and the output of the paralleled devices which limits the number of such devices which may be paralleled.
When using more than one amplifier because the required output exceeds the capability of a single device, such as a high frequency transistor, several amplifiers can be connected in parallel. There are disadvantages and dangers in the simple parallel connection. An input VSWR of 1.22, for example, represents a reflection power loss of only 1%. But if two devices both having a VSWR of 1.22 are connected in parallel, the power split between them depends on the impedance ratio which in this case could be as high as 1.5 if the phases of the two reflections were 180.degree. apart. Similar arguments can be made about output impedances. Not only is the power divided unequally, but if one unit fails because of unequal power split or for other reasons, the resulting high VSWR can adversely affect the remaining units.
A problem arises in prior art divider/combiner circuits that utilize integral damping resistors to provide isolation between paralleled solid state amplifiers. These integral damping resistors introduce instability and also reduce efficiency in the operating mode. Although, in the prior art divider/combiner circuits the isolation resistors are connected so that currents should not flow in the operating mode, the distributed reactances within the circuit do produce current flow in the isolation resistors in the operating mode; and, hence, the stability and efficiency of the divider/combiner circuit is reduced.
Although solid state power amplifiers using combined negative resistance diodes are becoming available for use in the 8-20 GHz frequency range, they have inherent problems with noise performance, limited dynamic range, and poor stability which limits their utility. Transistors are presently being developed which operate in this frequency range and have demonstrated 5 watts output with 5 db gain at 8 GHz and 1/2 watt output with 5 db gain at 20 GHz. Numerous applications are envisioned for solid state microwave amplifiers delivering 10-50 watts output power. Assuming 90 percent combining efficiency can be achieved, a reasonable number of existing transistors do provide the desired output power when combined as in this invention.